The long-range goal of this proposal is to continue to evaluate the potential role of neurokinin-1 receptor (NK1R) antagonists as anti HIV-1 agents in vivo. Aprepitant is the only FDA approved substance P antagonist. We have demonstrated in our previous work that NKIR antagonists, in general and aprepitant, in particular, have significant anti HIV-1 activity in vitro possibly mediated through CCR5 down-regulation, although some of our in vitro data suggests that these compounds have some antiviral activity in CXCR4 viruses. Aprepitant is metabolized primarily by CYP3A4 with minor metabolism by CYPIA2 and CYP2C19. This metabolic pathway suggests that aprepitant will interact favorably with the protease inhibitor ritonavir (a potent inhibitor of CYP3A4), as do many ofthe current available antlretrovirals. We will examine the safety and the PK characteristics of the NKI R antagonist, aprepitant, in HIV-infected subjects with well controlled viral replication receiving ritonavir containing antiretroviral therapy. Our hypothesis is that aprepitant will be safe, tolerable and that with the concomitant administration of the protease inhibitor ritonavir, we will be able to attain the therapeutic levels necessary to achieve antiviral activity predicted by both by our in vitro studies and our pharmacokinetic and pharmacodynamic (PK/PD) modeling. We will also examine the antiviral activity of an optimized dose of aprepitant administered for two weeks as an "add on" drug in patients with evidence of virologic failure on a protease inhibitor containing regimen. Further, we hypothesize that aprepitant will improve depressive symptoms, decrease anxiety and improve sleep quality in patients with HIV infection.